Where can I find Alston slide door shower door wheel parts?

Dear Sir/Madam,

Our Alstone Glass shower enclosure -(slide door model) the shower door wheel ~plastice wheel is broken. Where can find the spare parts for this? & order from - I cant find it online so far. I live in Glasgow.

Alstone shower door plastic wheel
Shower glass -code ARMOURTECH BS 6206 A T 1395

Please help! Thank You so much
Jo